HOW IS YOUR VOICE DELIVERED?

When you stand or sit, what is your posture like?
Is your back straight, hunched, slouched, caved in?
Do you put all your weight on one leg?

Always be aware of your posture as this can effect voice production.

BREATHING
Think of your airflow as being like petrol in a car. You need enough in the tank (lungs) to drive the car; it should flow through to the engine smoothly (air from lungs to vocal folds), and at a sufficient volume to enable the engine to work efficiently; and you be aware of when it is getting low and the tank needs a refill.

This point is very important. With correct breathing, your voice throw will also be more powerful and steady. Breathing exercises should be done regularly to ensure this. Ever gone through a stage in singing when you run out of breath no matter how deep a breath you took before commencing the line? It could be down to incorrect intake and releasing of breath. 
When we talk, we don't stop to take in breath before saying a line, similarly, breath should be controlled when singing.

Secondly, when you breath, have you ever considered the following:
Your posture?